Conventionally, there is presented a charging controller which is connected to at least one secondary cell and also detachably connected to a charger or a cell driver to control charging from the charger to the secondary cell.
Such charging controller includes a charging amount control circuit controlling a charger to charge up a cell voltage to a target voltage and a protection circuit preventing an overcharging. The charging amount control circuit outputs a charging control signal that instructs the charger to decrease a charging current when the cell voltage of any one of the secondary cells exceeds the target voltage. Further, the protection circuit prevents the overcharging by blocking a current path from the charger to the secondary cell when the cell voltage exceeds an upper limit voltage (see, e.g., Japanese Patent Application Publication No. 2007-143284)
In the conventional charging controller, both of the charging amount control circuit and the protection circuit is supplied a power from the charger. Therefore, if a circuit generating the power in the charger becomes out of order, the charging amount control circuit and the protection circuit cannot properly work, which may lead to an overcharging.
In this regard, there may be considered using a secondary cell as a power source in order to make certain that the charging amount control circuit and the protection circuit always work. However, with this case, even if the secondary cell is in a standby state where the charger and the cell driver are not connected, power of the secondary cell is unnecessarily consumed.